The paper seems to be making an assumption that the air is "well mixed." I don't see how that holds unless there is no ventilation whatsoever, in which case, C

The paper seems to be making an assumption that the air is "well mixed." I don't see how that holds unless there is no ventilation whatsoever, in which case, CO2 levels will build up to dangerous levels. In the two restaurant case studies I've seen with transmission beyond 6 feet, there is a clear effect of fans moving air from one person to another in the pattern of who gets infected and who doesn't. I haven't seen a documented case of transmission beyond 20 feet. Yes, it's probably technically possible to transmit COVID from a 60 foot distance, but the probability is much less, and it's inaccurate to say that you're no safer at 60 feet than 6 feet.